“PCB Layout is a relatively meticulous work, in which there are not only rules and constraints, but also many large and small matters needing consideration by engineers. In this article, Banermei has sorted out some details that need to be paid attention to in Layou, to check if you all know it!
PCB Layout is a relatively meticulous work, in which there are not only rules and constraints, but also many large and small matters needing consideration by engineers. In this article, Banermei has sorted out some details that need to be paid attention to in Layou, to check if you all know it!
1. Layout of special components
√ The heating element should be placed in a position conducive to heat dissipation, such as the edge of the PCB, and away from the microprocessor chip;
√ Special high frequency components should be placed next to each other to shorten the connection between them;
√ Sensitive components should be kept away from noise sources such as clock generators and oscillators;
√ The layout of adjustable components such as potentiometers, adjustable inductors, variable Capacitors, and key switches should meet the structural requirements of the whole machine and facilitate adjustment;
√ Components with heavier mass should be fixed with brackets;
√ The EMI filter should be placed close to the EMI source.
2. Crystal placement
√ The crystal oscillator is composed of quartz crystal, which is easily affected by external impact or drop. Therefore, it is best not to place it ON the edge of the PCB during layout, but as close to the chip as possible.
√ Place the crystal oscillator away from heat sources, because high temperature will also affect the frequency offset of the crystal oscillator.
3. Device decoupling rules
Add necessary decoupling capacitors on the printed plate to filter out the interference signal on the power supply and stabilize the power supply signal. It is recommended that the power supply be connected to the power pin after passing through the filter Capacitor.
4. Placement of decoupling capacitors for IC
A decoupling capacitor needs to be placed near the power port of each IC, and the location should be as close as possible to the power port of the IC. When a chip has multiple power ports, a decoupling capacitor should be placed at each port.
5. Electrolytic capacitors should be kept away from heat sources
When designing, the PCB engineer must first consider whether the ambient temperature of the electrolytic capacitor meets the requirements, and secondly, keep the capacitor as far away from the heating area as possible to prevent the liquid electrolyte inside the electrolytic capacitor from drying out.
6. Spacing between patches
The spacing between SMD components is a problem that engineers must pay attention to when laying out. The spacing between SMDs can neither be too large (wasting circuit layout) nor too small to avoid solder paste printing adhesion and welding repair difficulties.
The spacing size can refer to the following specifications:
√ Same device: ≥ 0.3mm
√ Different devices: ≥ 0.13×h+0.3mm (h is the maximum height difference between the surrounding neighbors and the device)
√ When manually soldering and patching, the distance between the device and the device is required: ≥ 1.5mm.
(For reference only, you can follow the PCB process design specifications of the respective companies)
7. Component lead width is the same
8. Reserve unused pin pads
For example, if two pins of a chip in the above picture are not used, but the actual pins of the chip exist, if the two pins are in a floating state as shown on the right side of the above picture, it is easy to cause interference.
If the chip pin itself is not connected (NC), adding the pad and then grounding the pad can avoid interference.
9. Be careful when using vias
In almost all PCB layouts, vias must be used to provide conductive connections between different layers. PCB design engineers need to be especially careful because vias create inductance and capacitance. In some cases, they also produce reflections because the characteristic impedance changes when vias are made in the trace.
Also keep in mind that vias increase trace length and need to be matched. In the case of differential traces, vias should be avoided as much as possible. If unavoidable, vias should be used in both traces to compensate for delays in the signal and return paths.
10. Barcode Screen printing settings
1) The barcode silk screen is placed horizontally/vertically.
2) The barcode position is based on the principle of not covering the pads, test holes, not being covered by the handle bar, and easy to read the information.
3) 5mm from the edge of the board and 15mm from the handle bar. The preferred placement sequence is as follows:
Barcode screen printing priority placement sequence diagram
4) Single-sided device board: Top surface solid line frame→Top surface dotted line box; double-sided device board: all solid line boxes.
5) The preferred order of barcode screen printing frame size: 42*8mm→42*6mm→7*9mm. 42*8 is suitable for veneer with automatic line.